Optical recording employing a laser makes the storage of high density information recording and its reproduction possible. Accordingly, its development has been remarkably proceeded in recent years.
As an example of an optical recording medium, an optical disc may be mentioned. In general, an optical disc is designed so that high density information recording is conducted by irradiating a laser beam focused to about 1 .mu.m to a thin recording layer provided on a disc-shape substrate. The recording is conducted in such a manner that upon absorption of the irradiated laser beam energy, such a portion of the recording layer undergoes a thermal deformation such as decomposition, evaporation or dissolution. Further, the reproduction of the recording information is conducted by reading the difference in reflectance between the portion where a deformation was formed by the laser beam and a portion where no such deformation was formed.
Accordingly, the recording layer is required to efficiently absorb the laser beam energy, and a laser-absorbing dye is employed.
Various constructions have been known for optical recording media of this type. For example, Japanese Unexamined Patent Publication No. 97033/1980 discloses a medium having a single layer of phthalocyanine type dye provided on a substrate. However, the phthalocyanine type dye has a problem that the sensitivity is low, and the decomposition point is high and vapor deposition is difficult. Further, it has an additional problem such that the solubility in an organic solvent is very poor, whereby it can not be used for coating in the form of a coating solution.
On the other hand, Japanese Unexamined Patent Publications No. 112790/1983, No. 114989/1983, No. 85791/1984 and No. 83236/1985 disclose media having cyanine-type dyes as the respective recording layers. Such dyes have high solubility and thus have a merit that coating in the form of a coating solution is possible. However, they also have a problem that they are inferior in the light resistance. In this connection, Japanese Unexamined Patent Publication No. 55795/1984 proposes to improve the light resistance by an addition of a quencher to such a cyanine type dye. However, such a proposal is still at an inadequate level.
In connection with such problems, Japanese Unexamined Patent Publication No. 30090/1987 discloses a recording medium wherein a complex of a monoazo compound with a metal, is employed, as a recording medium having the solubility in an organic solvent and the light resistance improved. However, such a compound is inferior in the sensitivity with the light sensitive wavelength being short, and further it is inferior in the storage stability at a high temperature high humidity condition, whereby it has problems as an optical recording medium.